1. Field of the Invention
The present invention relates to an electric power steering apparatus that uses an electric motor as an auxiliary power source for steering, and more particularly to a power steering apparatus in which the torque of the electric motor is transmitted to a steering mechanism via a worm gear-type reduction mechanism.
2. Description of Related Art
A power steering apparatus is provided to apply steering assisting power to the steering mechanism in accordance with the steering torque applied at the steering wheel, thereby reducing the effort required to steer the road wheels and thus achieving a comfortable steering feel. In recent years, power steering apparatus have come to be used widely not only in large vehicles such as buses and trucks but also in smaller vehicles such as standard-sized passenger cars, mini cars, etc.
Power steering apparatus for this application are largely classified into two types according to the steering assisting power source used: the hydraulic type that uses a hydraulic actuator such as a hydraulic cylinder; and the electric type that uses an electric motor. The electric-type power steering apparatus is constructed to provide steering assisting power by driving a steering assisting electric motor installed midway through a steering mechanism in accordance with the result of the detection of the steering torque applied at the steering wheel, and by applying the torque of the motor to the steering mechanism. The use of this type of power steering apparatus has been increasing recently because of the advantage that the steering assisting power characteristics can be easily changed by controlling the energization of the electric motor according to driving conditions such as vehicle speed, steering frequency, etc.
One problem with the electric power steering apparatus has been the difficulty in obtaining a compact electric motor capable of providing sufficient torque for steering assisting power, coupled with the difficulty in securing space in a vehicle for installation of such a motor. This problem, however, has been solved by installing a reduction mechanism midway through a steering power transmission system coupling from the power assisting motor to the steering mechanism to reduce the motor speed thereby increasing the motor torque for transmission to the steering mechanism. The reduction mechanism is required to be constructed compact in size to reduce the overall size of the transmission system, while providing a high reduction ratio. To meet this requirement, a worm gear-type reduction mechanism is widely used which has a worm connected to the output side of the steering assisting electric motor, and a worm wheel that engages with the worm and is fitted onto the output shaft connected to the steering mechanism.
Another problem with the electric power steering apparatus is that if the power assisting motor is locked, the steering mechanism coupled to the output side of the motor via the reduction mechanism is thrown into a locked condition, losing steering maneuverability. Accordingly, safety measures to prevent the occurrence of steering maneuverability loss are essential. To achieve this, it has been widely practiced to interpose an electromagnetic clutch between the steering assisting motor and the reduction mechanism, the electromagnetic clutch being shut off in the event of a motor lock to disengage the motor from the steering mechanism.
The power steering apparatus of the above construction, however, has the disadvantage that the interposition of the electromagnetic clutch prevents the reduction of the overall size of the transmission system that may be achieved by the reduction in size of the motor and the reduction mechanism. To overcome this disadvantage, Japanese Patent Application Laid-Open No. 2-120178 (1990) and Japanese Utility Model Application Laid-Open No. 2-15576 (1990) disclose power steering apparatus in which a mechanical torque limiter, designed to cause slippage upon the application of an excessive rotational torque and thereby disengage the coupling between the steering assisting motor and the steering mechanism, is provided in the reduction mechanism between the motor and the steering mechanism, thereby preventing the occurrence of steering maneuverability loss caused by a motor lock without using an electromagnetic clutch.
According to the construction disclosed in the former patent publication number, one of the gear wheels forming the reduction mechanism is fitted in freely rotatable fashion on a support shaft, and a plurality of clutch plates whose rotation is restricted by the gear wheel and the support shaft, respectively, are alternately mounted one behind another inside the gear wheel, the gear wheel being allowed to rotate freely upon the application of a rotational torque that exceeds the friction engagement torque between these clutch plates. This construction, however, merely replaces the electromagnetic clutch by a friction clutch, and requires the provision of a special gear wheel having an internal construction too complex for general use.
On the other hand, the power steering apparatus disclosed in the latter patent publication number employs a simple construction; that is, while one of the gear wheels forming the reduction mechanism is fitted in freely rotatable fashion on a support shaft, as in the former case, one side face of this gear wheel is pressed by a pressure spring to press the other side face against a step formed on the support shaft, with a friction plate interposed between the pressing face and the face pressed by the pressure spring, allowing the gear wheel to rotate freely with the friction plate being caused to slip upon the application of an excessive rotational torque.
In the latter construction, however, the free rotation of the gear wheel is accompanied by slippage on the fitting face on the support shaft, and since this slippage is dependent on the fitting gap between the two parts, the fitting portions of the gear wheel and support shaft require high machining accuracy to ensure a reliable operation as a torque limiter, the resulting problem being an increase in the number of machining steps. The dependence of the slippage on the fitting face can be eliminated by presetting the fitting gap on the larger side, but this gives rise to the problem that the gear wheel is caused to rotate with radial displacement, adversely affecting the engaging condition on its outer circumference with other gear wheels and thus preventing proper torque transmission that the gear wheel is supposed to carry out.
Furthermore, the latter construction is specifically designed for application to spur gears, and if the same construction is applied to the worm wheel in the earlier described worm gear-type reduction mechanism, there is a possibility that, because of the axial force acting on the engaging face with the worm, the worm wheel may be displaced in axial direction against the urging force of the pressure spring, making it impossible to maintain proper engagement with the worm. Moreover, since the spring force of the pressure spring varies under the influence of the axial force, it is difficult to precisely set the upper limit torque above which the worm wheel is allowed to rotate freely; the resulting problem is that free rotation of the worm wheel may occur when the power steering apparatus is operating normally, resulting in a loss of proper steering assisting power and thus causing a sudden change in the steering feel. Further, there is a possibility that the worm wheel may not be placed in a freely rotatable condition in time in the event of a motor lock, causing a safety problem such as the occurrence of steering maneuverability loss in transition.